The present invention relates to treatment of organic material, grown for energy production or organic waste, which is slurried, suspended or dissolved in liquid, in a two-step anaerobic biochemical reactor configuration comprising recirculation between the two reactors. The organic material is fed to a first reactor in which hydrolysis and dissolution take place resulting in formation of volatile fatty acids. Liquid is transferred from the first reactor to the second, methanogenesis reactor where the treatment is finalized.
In processes as defined above, the organic waste is fed to a first reactor in which hydrolysis and dissolution of particulated material to carbohydrates, oligopeptides and fatty acids takes place. Volatile fatty acids (VFA) are formed when the hydrolysis products are broken down. In the second reactor the VFA are broken down to acetate and methane where methane is produced by cleavage of acetate or reduction of carbon dioxide by means of hydrogen. The microbial activity in each stage depends on specific environmental conditions with regard to temperature, pH, availability of substrate and inhibiting substances. One group of such inhibiting substance with regard to the methanogenesis and also hydrolysis is VFA. A major problem in such processes is therefore to control and/or adjust the presence of VFA in the two reactors in order to obtain an efficient process with regard to conversion of the organic material to methane.
Two-step anaerobic biological processes are generally known in the art. Thus it is known from Forschungsprojekt Nr. 2020, Forschungsbericht arbi, Arbeitsgemeinschaft Bioenergi 8933 Maschwanden, a two-step anaerobic digestion process for treatment of organic solid waste. The process permits separation of the solid and liquid phase allowing the operation of high rate anaerobic methanogenesis for the liquid. Circulating water is used for extraction of VFA and other components which are dissolved in the hydrolytic step. The water is treated in the methanogenic step in a pulsating dynamic filter before being recirculated to the hydrolytic step. Over 90% of the methane is generated in the methanogenic step. More than 80% of easily degradable organic waste is converted into biogas in less than 5 days.
The rate limiting stage will often be the hydrolyses and special measures must therefore be made. As the organic material is mainly solids, the formation of VFA will depend on the particle size of said solids. The special provisions for regulating the VFA will cause special problems.
Further there is described a two-phase anaerobic wastewater treatment system in Water Research Vol. 28, No. 2 pp 475-482 1994. The overall performance of the system was determined as a function of the extent of recycle of the effluent from the methanogenic reactor to the acidification reactor. The acidification stage was controlled at pH 6 by automatic addition of caustic soda, and a sand bed reactor for the methanogenic stage which was left uncontrolled. The required use of caustic soda will make such a process expensive.
Treatment of grass-clover silage in an anaerobic two-phase biogas process is further known from Antonie van Lecuwenhoeck 68: pp 317-327, 1995. Also in this process liquid recirculation was applied. Increased acetate in both reactors after initiating liquid recirculation was observed. It was found that liquid recirculation in silage-fed two-phase biogas processes will stimulate the activity of hydrogenotrophic methanogenesis in the liquification-acidogenic reactor leading to more thermodynamically favourable conditions for acidification reactions which are dependent upon interspecies transfer of reducing equivalents. Also this process utilizes solid organic material in the hydrolysis reactor. A further disadvantage of this process is that it is controlled on volume basis only.
The main object of the invention was to arrive at an improved two-step anaerobic biological process for treatment of organic material giving higher yield with regard to conversion of the organic material and methane production.
Another object was to arrive at an optimal process, especially with regard to VFA content in both reactors.
A further object was to obtain optimal pH-ranges for the processes in both reactors without use of pH regulating chemicals.
During further studies of the processes in the two reactors it was found that the simplest way of controlling and optimizing the operating conditions in both reactors might be obtained by changing the conventional flow of liquid/substrate between the two reactors. Though the invention is described with regard to application in a two-reactor system, it will be within the scope of the invention to utilize the same principle in a multi reactor system. It was found that the recirculation of the liquid phase between the two reactors could be performed according to various principles. The simplest principle (Q1) seemed to be applying a fixed recirculation rate independent of other parameters in the reactors. However, the organic load in a biogas system will not be constant, and the system will therefore to some extent be dynamic. Accordingly it was found that a fixed recirculation rate would not be desired.
The inventors then tried to apply another principle (Q2) which comprised changing the degree of recirculation in view of obtaining optimal operating conditions in both reactors and thereby obtain an overall improved efficiency of the process. The main reason for applying recirculation will be control of VFA and pH in the system. Reliable monitoring of the VFA content was found to be most difficult, but pH in the system will be influenced by the VFA content, and it was decided to rely on pH measurements as steering/controlling parameter. Though this approach seemed promising, it was found that operating at fixed optimal pH values in the respective reactors for acidogenesis and methanogenesis, known from the literature, would not constitute the optimal solution to the problem. It was then decided to switch the recirculation off and on for selected periods (Q3). This could for instance be performed by switching the recirculation on when the pH in the first reactor reaches a pre-determined lower level. When the pH passes the upper level the recirculation will be switched off. It may alternatively be switched off after a predetermined time period. Oscillation of pH and VFA-concentration in the reactor will thereby be attained. The recirculation may alternatively be switched off after a predetermined time period. The VFA concentrations in the reactors can also be controlled in a similar way by applying conductivity measurements or other suitable ways of measuring or detecting the influence of varying the amount/concentration of VFA in the reactors. Methane production (m3/h) has for instance been found to be a useful parameter for controlling the process. By allowing the VFA concentration to oscillate, the recirculation will pulsate at a variable rate according to the load of organics in the reactors.
The various ways of performing the recirculation, as described above, are shown in FIG. 2.
The main feature of the method according to the invention is that the amount of volatile fatty acid in each reactor is forced to oscillate within pre-set upper and lower limits by pulsed recirculation between the two reactors.
The amounts of VFA in the reactors can be controlled by varying the recirculation rate between the reactors.
The pH or conductivity in the liquid in the reactors can be monitored and used for calculating and controlling the amount of VFA in both reactors. The pH in each reactor can be allowed to vary within pre-set values.
Principally, any parameters which indicate variations of VFA concentrations and/or pH in the respective reactors could be monitored such that they can be used for calculating and controlling the VFA in the reactors.
The scope of the invention and its special features are as stated in the attached claims.